You're idling in the fast-food drive-thru, gazing at the giant lit-up menu. A voice crackles from the speaker, asking for your order. What do you choose?

If you opt for the combo meal, watch out: You might end up eating more than you really want or need.

It turns out that people tend to consume more calories when they order a combo meal or value meal off a fast-food menu than when they order items separately, according to a study in the current issue of the Journal of Public Policy & Marketing.

That's because combo meals (also called bundles) include items, such as french fries, we otherwise might not order, and because the sides and drinks in combo meals tend to be larger than what the general population would order a la carte.

In the study, 215 American adults imagined they were visiting nine fast-food outlets on a cross-country road trip. The participants were asked to order from different menus, which either included bundled meals (burger, fries and a drink) or offered the same items a la carte.

Fifteen percent of participants who passed on the fries on an a la carte menu ended up buying fries when a bundled meal was available. Twenty-six percent of those who bought fries a la carte increased their order size of fries when a bundled meal was offered, consuming more than 100 extra calories on average.

Why do so many of us order combo meals? The motivation goes beyond the desire for a good value, according to Sharpe.

“It's cognitively easier to say, ‘I just want a No. 1,'” she says. The combo meal option “also signals that this might be an appropriate size for a meal for the average consumer.”

These findings could have public policy implications, according to the authors.

Sharpe doesn't expect fast-food chains to stop offering combo meals, but why not introduce policies providing incentives for restaurants to cut calories in those meals by bundling, say, a 16-ounce soda rather than a 21-ounce soda?

That might not seem like a big change, but an extra 5 ounces of soda a day can add up to about a 5-pound weight gain in a year.

Policy-wise, shrinking the size of combo meals would be a more effective way to cut calorie consumption than taxing fast food, the study's authors say, and would have a minimal impact on profits.

Until that happens, you might want to follow Sharpe's lead at the drive-through.

“I always unbundle and I always choose the smallest,” she says. “And generally I'm satisfied. Research shows that people are generally averse to things that are the smallest because they're concerned they might not get enough, but what is the smallest now used to be the largest. So they will get enough.”